The disclosure of Japanese Patent Application No. 2001-228326 filed on Jul. 27, 2001 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a shift control device and a shift control method for an automatic transmission of a vehicle. More particularly, the invention relates to an art which, in a multiple downshift that involves a second downshift starting before a first downshift is completed, suppresses a shift shock by accurately executing learning for the second downshift.
2. Description of the Related Art
There is known, for example, a shift control device for a vehicular automatic transmission provided with a first downshift control portion that executes a first downshift from an nth speed to an (nxe2x88x921)th speed, while performing a torque down for suppressing shift shock, a second downshift control portion that executes a second downshift from the (nxe2x88x921)th speed to an (nxe2x88x922)th speed or an (nxe2x88x923)th speed, and a learning control portion that performs a learning control for stabilizing a downshift operation when the downshift from the (nxe2x88x921)th speed to the (nxe2x88x922)th speed or the (nxe2x88x923)th speed is executed by the second downshift control portion.
An example of such a device is a downshift control device for a vehicular automatic transmission, to which a learning control for stabilizing downshift operations is added, as disclosed in Japanese Patent Publication No. 5-43528. According to this arrangement, in the first downshift from the nth speed to the (nxe2x88x921)th speed executed by the first downshift control portion, an engine output torque or an input torque to an automatic transmission is temporarily reduced, so that a shift shock is reduced. In addition, in the second downshift from the (nxe2x88x921)th speed to the (nxe2x88x922)th speed or the (nxe2x88x923)th speed executed by the second downshift control portion, individual differences and time-dependent changes in friction engagement characteristics are assimilated, which ensures a stabilized downshift operation even in the case of a clutch-to-clutch shift.
In such a downshift control device for a vehicular automatic transmission described above, a multiple downshift in which the second downshift starts before the first downshift is completed may be executed. In such a multiple downshift, because of the torque down due to the first downshift, the time required for engagement of a hydraulic friction engagement device in the second downshift becomes shorter as compared with the case where the second downshift is performed independently. Therefore, the engagement condition of the hydraulic friction engagement device, which serves as a prerequisite for the learning control, is affected. Thus, erroneous learning may be performed, and accordingly the second downshift may become unstable.
It is therefore an object of the invention to provide a downshift control device for a vehicular automatic transmission, which ensures that erroneous learning is not performed due to the torque down for the first downshift during a multiple downshift in which a second downshift starts before a first downshift is completed.
To achieve the foregoing object, a shift control device for a vehicular automatic transmission according to a first aspect of the invention includes a first downshift control portion that executes a first downshift, while performing a torque down for suppressing a shift shock; a second downshift control portion that executes a second downshift from a gear position reached in the first downshift; and a learning control portion that performs a learning control for stabilizing the downshift operation when the second downshift is executed by the second downshift control portion. The shift control device also includes a multiple downshift determination portion that determines whether or not the current downshift is a multiple downshift in which the second downshift is started before the first downshift is completed; a learning interference determination portion that determines whether or not the torque down during the first downshift is in a condition of interfering with the learning performed by the learning control portion when the multiple downshift determination portion determines that the current downshift is the multiple downshift; and a learning prohibition portion that prohibits the learning performed by the learning control portion if the learning interference determination portion determines that the torque down during the first downshift is in the condition of interfering with the learning performed by the learning control portion.
According to the first aspect, if the multiple downshift determination portion determines that the current downshift is a multiple downshift, the learning interference determination portion determines whether or not the torque down during the first downshift is in a condition of interfering with the learning performed by the learning control portion. If the learning interference determination portion determines that the torque down during the first downshift is in the condition of interfering with the learning performed by the learning control portion, the learning prohibition portion prohibits the learning performed by the learning control portion. Therefore, erroneous learning is suitably prevented from being performed due to the torque down during the first downshift. As a result, the second downshift is suitably prevented from becoming unstable due to the erroneous learning, and thus good learning accuracy is maintained.
Further, the learning interference determination portion may determine that the torque down during the first downshift is in a condition of interfering with the learning performed by the learning control portion based on the fact that the first downshift has not been completed even when an elapsed time after the second downshift is started exceeds a predetermined value. This arrangement makes it possible to determine easily and accurately a condition in which the torque down during the first downshift interferes with or affects an engagement condition of the hydraulic friction engagement device that is involved with the second downshift.
Also, there may be further provided a learning variation determination portion that determines whether or not variations in the learning performed by the learning control portion for the second downshift are greater than a predetermined value, and a torque down prohibition portion that prohibits the torque down in the first downshift if the learning variation determination portion determines that the variations in the learning in the second downshift are greater than the predetermined value. According to these arrangements, the torque down prohibition portion prohibits the torque down in the first downshift if the variations in the learning in the second downshift are greater than the predetermined value. This promotes the learning in the second downshift, thereby quickly stabilizing the second downshift that greatly affects a shift shock.
Also, there may be still further provided a learning completion determination portion that determines whether or not a period of the learning performed by the learning control portion for the second downshift has been completed, and a torque down prohibition portion that prohibits the torque down in the first downshift if the learning completion determination portion determines that the period of the learning performed by the learning control portion for the second downshift has not been completed. According to these arrangements, the torque down prohibition portion prohibits the torque down in the first downshift if the period of the learning performed for the second downshift has not been completed. This also promotes the learning in the second downshift, thereby quickly stabilizing the second downshift that greatly affects a shift shock.
A shift control method for a vehicular automatic transmission according to a second aspect of the invention includes the steps of executing a first downshift, while performing a torque down for suppressing a shift shock; executing a second downshift from a gear position reached in the first downshift; performing a learning control during an execution of the second downshift for stabilizing the downshift operation; determining whether or not the current downshift is a multiple downshift in which the second downshift is started before the first downshift is completed; determining whether or not the torque down during the first downshift is in a condition of interfering with the learning when it is determined that the current downshift is the multiple downshift; and prohibiting the learning if it is determined that the torque down during the first downshift is in the condition of interfering with the learning.
According to the second aspect, if it is determined that the torque down during the first downshift is in the condition of interfering with the learning, the learning is prohibited. Therefore, erroneous learning is suitably prevented from being performed due to the torque down during the first downshift. As a result, the second downshift is suitably prevented from becoming unstable due to the erroneous learning, and thus good learning accuracy is maintained.